Incremental optical motion encoders are used for resolving the position or measuring rotation of a shaft. Exemplary shaft angle encoders are known from U.S. Pat. Nos. 4,451,731 and 4,691,101, for example. Such encoders include a light source for emitting a light beam, an encoder wheel for modulating the light beam in response to shaft rotation, and a detector assembly for receiving the modulated light and producing electrical signals indicating the amount of light received by the detectors.
As the light is modulated in response to shaft rotation, each electrical signal from the detector assembly produces a wave form. The position of the shaft determines the position of each signal on its particular wave form, that is, the phase of each signal. Thus, the electrical signals from the detectors can be used to indicate shaft rotation. Two or more properly out-of-phase signals, from separate detectors, can be used to indicate both direction and magnitude of rotation.
Typical shaft angle encoders have an opaque wheel with a plurality of transparent windows through which illumination passes. In the simplest embodiment, a light source is provided adjacent to one face of the wheel. A photodetector is placed on the opposite side of the wheel so that as the wheel rotates, light from the emitter is alternately passed through a window to the photodetector or occulted by an opaque area of the wheel. There may be a number of drawbacks to such an arrangement, particularly when relatively low-cost encoders are desired.
Much of this can stem from having the light emitter on one side of the wheel and the light detectors on the opposite side of the wheel. This means that there are two substrates to which electrical connections must be made. Having two substrates complicates the positioning and alignment of components to obtain a desired precision. Each of these adds significant cost to the encoder.
Further, to obtain precision and high contrast, it is commonly necessary to employ collimating optics for the light source. This adds an element of complexity and difficulty in assembly alignment and also enlarges the size of the encoder. In the modern world, it is almost universal that small size is desirable.
It is, therefore, desirable to produce a shaft angle encoder where the light emitters and photodetectors can be mounted on a single substrate.